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Illustration of amyotrophic lateral sclerosis (ALS), also referred to as Lou Gehrig's disease. ALS is the most common of the five motor neuron diseases. On the left is shown a normal neuron and muscle interface. On the right is shown an afflicted neuron. Nerve degeneration and restricted production of the neurotransmitter dopamine results in deterioration of muscle tissue. This debilitating disease is characterized by rapidly progressive weakness, muscle atrophy and twitching, muscle spasticity, difficulty speaking (dysarthria), difficulty swallowing (dysphagia), and difficulty breathing (dyspnea).